In recent years the use of high strength-to-weight ratio fiber reinforced resin composites has continuously expanded, particulary in weight sensitive products, such as aircraft and space vehicles. Fiber reinforced resin composites are usually formed by "laying-up" a plurality of plies formed of reinforcing fibers. The plies may be preimpregnated with resin; or, resin layers may be added to stacks of dry plies as the lay-up is being formed. In any event, after a lay-up is formed, the resin is cured by applying heat and, usually, pressure to the lay-up. When dry plies and resin layers form the lay-up, the resin is infused into the dry plies as it is cured. In the past, a wide variety of fiber reinforced resin composite components, such as panels, reinforcing members and combinations thereof, have been formed in this manner.
As the use of fiber reinforced resin composites has expanded, the desire to maintain weight at a minimum commensurate with structural integrity has increased the complexity of the resulting structures. In the past, complex structures have been either laid up and cured as a single item or separately formed and joined by mechanical fasteners, such as rivets. Both of these approaches have disadvantages. Complex structures have the disadvantage that they are difficult to layup. Using mechanical fasteners to join previously created fiber reinforced resin composite reinforcing members and panels has the disadvantage of creating joints that tend to weaken as vibration causes wear about the periphery of the holes in which the fasteners are mounted.
As the complexity of fiber reinforced resin composites has increased other problems have also developed. These problems are primarily a result of the ply by ply nature of resin composites, which makes them susceptible to delaminate along interlaminar planes. The tendency to delaminate is, of course, a result of the fact that most composites do not have fibers in the through thickness direction (i.e., the direction orthogonal to the planes of the plies that form the composite), whereby all resistance to delamination along interlaminar planes is a function of the properties of the resin, which usually has a strength 50 to 100 times less than the ply fibers, or a comparable substitute metal.
Several attempts have been made to solve the delamination problem noted above. One attempt involves modification of the chemistry of the resins used in fiber reinforced resin composites to increase resin fracture toughness while maintaining composite compression strength. Most methods of increasing the fracture toughness of a brittle resin, such as an epoxy, involve the addition of at least one component with a lower shear modulus than the resin base. The disadvantage of this approach is that this addition results in a drop of the overall shear modulus. In environments where this approach has been tried, namely in connection with the manufacture of components for aircraft and space vehicles, the shear modulus has dropped below that required to maintain composite compression performance. This approach also has the disadvantage of increasing the susceptibility of the resultant fiber reinforced resin composite to heat and solvents.
A further approach to solving the tendency of fiber reinforced resin composites to delaminate has been to add more mass to keep stresses below the maximum dictated by a low delamination resistance. This approach has the disadvantage of substantially reducing the weight saving advantages of fiber reinforced resin composites.
Another approach used to solve the delamination problem is to design discrete portions of a composite to have a substantially higher stiffness than the remainder of the composite. The stiffener portions become the primary load carrying paths and can be varied to meet structural requirements. The disadvantage of this approach is that it places severe restrictions on design. Further, weight is increased by the material added to discretize the stiffness of various load paths. The end result is a structure that is heavier and more difficult to manufacture than comparable structures that do not have these design constraints.
Finally, the delamination resistance of fiber reinforced resin composites along interlaminar planes has been improved by cross-ply stitching laminate plies together. While this approach does not substantially increase the weight of the resultant reinforced resin composite, it is difficult to satisfactorily implement in fiber resin reinforced composites having complicated cross-sectional configurations.
The present invention is directed to delamination resistant fiber reinforced resin composites that avoid the disadvantages described above.